Corneal Allogenic Intrastromal Ring Segments

An Alternative to DALK in Certain Situations

Soosan Jacob, MS, FRCS, DNB, MNAMS

Intrastromal corneal ring segments (ICRS) have been used in the past to treat keratoconus and other ectatic disorders. It is a reversible procedure that flattens the central corneal curvature while leaving the central visual axis of the cornea unaltered. The segments have an arc shortening effect and bring about a regularization in corneal topography; however, in the past, these have been made of synthetic material and have been reported to have up to a 35% complication rate¹ associated with implantation of synthetic substance within the cornea, such as corneal haze, channel deposits, corneal neovascularization, and chronic pain, as well as more serious complications, such as extrusion, erosion, migration, and infectious keratitis.

Corneal allogenic intrastromal ring segments (CAIRS) is a new procedure that was devised by the author and is similar to Intacs (Addition Technology Inc), Kerarings (Mediphacos Inc) but with the distinct advantage of using allogenic tissue^2,3 (Figure 13-1).

INDICATIONS

Keratoconus, pellucid marginal degeneration, other causes of corneal ectasias, and irregular corneas are indications for CAIRS implantation. They are indicated in patients with mild to moderately severe disease and unsatisfactory vision with glasses. A clear cornea over the visual axis is a prerequisite.

CAIRS produces a flattening and regularization of the corneal topography and, therefore, decreases irregular astigmatism, reduces corneal steepening, and improves uncorrected visual acuity (UCVA) and best-corrected visual acuity (BCVA) while also decreasing the spherical equivalent, sphere, and cylindrical components of the refractive error (Figure 13-2). Similar to synthetic segments, its effect depends on the thickness of the segment as well as the diameter and optic zone. The cornea follows the Barraquer and Blavatskaya postulates, which state that peripheral addition in the cornea causes flattening, and the diameter of the implanted ring determines the degree of flattening. Therefore, increasing the amount of tissue added, as well as decreasing the diameter, both cause a greater degree of myopia correction.⁴ CAIRS implantation is also likely to have an additional beneficial biomechanical effect secondary to a redistribution of stress forces.

Screened and processed donor cornea is used to create corneal segments that are then implanted into the patient’s cornea. Non-edematous donor corneal rims with negative serology for anti-HIV-1, anti-HIV-2, Hepatitis B surface antigen, anti–Hepatitis C virus, and venereal disease research laboratory test is utilized. The quality of the endothelium is not important as it is not transplanted. The donor cornea is first mounted on an artificial anterior chamber and epithelium removed completely. The center is then marked and laid upside down on a Teflon block, and the endothelium is removed as well. A double-bladed trephine designed by the author is then used to punch a segment of tissue from the donor cornea. This segment is then cut into 2 and soaked in riboflavin. Femtosecond laser–dissected channels are prepared in the patient’s eye and the CAIRS segments are inserted into these channels either using the CAIRS inserter designed by the author or using curved 23-gauge forceps. In patients where progression is likely, this procedure is combined with corneal cross-linking (CXL) whereas in older patients, where only a regularization of topography is required, it can be performed in isolation. If indicated, CXL, either conventional or contact lens–assisted corneal cross-linking (CACXL; depending on the minimum corneal thickness) may be performed, either simultaneously or sequentially following CAIRS implantation (Figures 13-3 to 13-7).

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